The disclosed technology regards a cooling system which can function to provide power plant condensers with cooling water at desirable temperature levels to maintain turbine power production at optimum thermal efficiency levels. The technology may also replace the power plant condenser, and provide the power plant low-pressure-turbine with return water at temperatures to achieve the turbine's designed optimum back pressure at any ambient conditions. The disclosed technology further relates to an improvement in dry cooling systems to overcome the inherent thermodynamic performance penalty of air-cooled systems, particularly under high ambient temperatures. The disclosed technology has other applications, including providing cooling and heating in air conditioning systems, and generally in the removal of heat from liquid sources in a controlled environment, as well as streams or other water sources in the natural environment. Using the methods of the technology, heat generated by the system may also be used to warm an environment or another liquid source.
More than 86% of electricity in the United States of America is produced by thermoelectric power generating plants, most of which use coal, natural gas, or nuclear fuel to generate thermal energy. As shown in FIG. 1, the thermal energy produces superheated steam in the boiler/steam generator, which drives a steam turbine to produce electrical power by the generator. Each power plant is designed for the conditions of its particular geographic location, which conditions impact the design point of the low pressure turbine exhaust pressure. The exhausted steam coming out of the turbine last stage is condensed in a condenser by cooling heat transfer with the condenser, then pumped back to the boiler as boiler return water, and the process is repeated. Although unique to each plant, the return condensate water ranges in temperature from 35° C. to 52° C.
The pressure of the outlet steam causing the turbine blade rotation, called back pressure, is defined by the condenser temperature. For dry cooling systems, the condenser temperature is a strong function of the ambient temperature. Therefore, an increase in ambient temperature directly affects the power plant efficiency. For indirect cooling systems, the ambient air increases the cooling water temperature which in turn increases the condenser temperature. However, for direct air cooled systems the condenser temperature is directly influenced by the ambient temperature.
Typically, more than 60% of the original energy generated by the steam generator/boiler is wasted and carried away as low-grade heat by the plant condenser cooling water or directly dissipated to the ambient air. Operators must remove this heat, and 99% of baseload thermoelectric plants in the United States of America use water-cooled systems, or wet cooling, to remove the heat from the condenser cooling water. Power plant operators prefer wet cooling over dry-cooling systems because ambient water temperatures tend to be cooler and more stable than ambient air temperatures; further, water evaporation allows for additional cooling capacity, enabling more cost-effective rejection of heat. However, the wet cooling processes lead to a significant amount of water loss, with power plants using wet-cooling systems currently accounting for 41% of all fresh water withdrawals in the United States of America.
Availability of fresh water resources is increasingly strained by drought and growing demands, and potential climate change impacts add uncertainty to the quality and quantity of future water supplies. However, while dry-cooling technologies do not result in significant water use, because of their sensitivity to ambient air temperatures current dry-cooling technologies drive down the overall efficiency of power generation compared with the efficiency of wet cooled condensers. Therefore, there is a need for a dry-cooling technology that eliminates water loss or the dependency on water while maintaining the high operating efficiencies of electric power generation presently achieved by wet-cooling technologies.
Power plant condenser cooling is divided into five main technology areas, which differ greatly in the amounts of water consumed: (1) once-through cooling; (2) closed-cycle wet cooling; (3) cooling ponds; (4) dry cooling; and (5) hybrid cooling.
Once-through cooling systems withdraw cold water from, and return heated water to, a natural body of water such as a lake, a river, or the ocean. In operation, the source water is pumped through the tubes of a steam condenser. As steam from the turbine condenses on the outside of the tubes, the heat of condensation is absorbed by the source water flowing through the tubes. The source water exiting the condenser, warmed by 15° F. to 30° F. depending on system design, is discharged to the original source. The amount withdrawn varies from 25,000 to 50,000 gallons/MWh. Although none of the water is consumed within the plant, some consumptive loss results from enhanced evaporation from the surface of the natural body of water due to the heated water discharge. The loss due to this enhanced evaporation is not well known and is expected to be site-specific, but it has been estimated as 0.5% to 2% of the withdrawn source water amount, or 125 to 1000 gallons/MWh. The biggest drawback of once-through cooling systems is that heated discharges may degrade the natural body of water, increasing the overall water temperature of the natural body of water. The thermal pollution is most significant when the source of the water is a river or other body with limited volume, where the water withdrawn and discharged is a significant portion of the natural water flow.
Closed-cycle wet cooling is similar to once-through cooling in that as cold source water flows through the tubes of a steam condenser, steam from the turbine condenses on the outside of the tubes. However, instead of returning the heated condenser water to its source, it is pumped to a wet cooling device such as a cooling tower, cooling pond, or cooling canal, where it is cooled by evaporation of a small portion of the water to the atmosphere to within 5° F. to 10° F. of the ambient wet-bulb temperature. Makeup water is added to compensate for the water loss due to evaporation and the again cooled water is then recirculated to the steam condenser.
Wet cooling devices used in closed-cycle wet cooling transfer thermal energy from heated cooling water to the atmosphere through both heat transfer to the ambient air and evaporation, to bring the cooling water to near wet-bulb air temperature. Specifically, as ambient air is drawn past a flow of cooling water, a small portion of the water evaporates, and the energy required to evaporate that portion of the water is taken from the remaining mass of water, thus reducing its temperature. About 970 Btu of thermal energy is absorbed for each pound of water evaporated.
To achieve better performance, heated cooling water may be sprayed to a medium, called fill, to increase the surface area and the time of contact between the air and water flow. Some systems use splash fill, which is material placed to interrupt the water flow causing splashing. Other systems use film fill, which includes thin sheets of material (usually PVC) upon which the water flows, enhancing evaporation.
Cooling towers draw air either by natural draft or mechanical draft, or both. Natural draft cooling towers utilize the buoyancy of warm air, and a tall chimney structure. In this structure the warm, moist air naturally rises due to the density differential compared to the dry, cooler outside air, producing an upward current of air through the tower. Hyperbolic towers have become the design standard for natural draft cooling towers due to their structural strength and minimum usage of material. The hyperbolic shape also aids in accelerating air flow through the tower, and thus increases cooling efficiency. Mechanical draft towers use motor-driven fans to force or draw air through the towers, and include induced draft towers which employ a fan at the top of the tower that pulls air up through the tower (as shown in FIG. 1), and forced draft towers which use a blower-type of fan at the bottom of the tower, which forces air into the tower.
Cooling ponds are man-made bodies of water which supply cooling water to power plants, and are used as an effective alternative to cooling towers or once-through cooling systems when sufficient land, but no suitable natural body of water, is available. The ponds receive thermal energy from the heated condenser water, and dissipate the thermal energy mainly through evaporation. The ponds must be of sufficient size to provide continuous cooling, and makeup water is periodically added to the pond system to replace the water lost through evaporation.
Current dry cooling systems can be a direct system, in which turbine exhaust steam is condensed in an air-cooled condenser (ACC), or an indirect system, in which the steam is condensed in a conventional water-cooled condenser. For indirect systems, the heated cooling water is circulated through an air-cooled heat exchanger before returning to the water-cooled condenser. In the direct system, the steam is condensed in the ACC in finned tube bundles (galvanized steel tubes with aluminum fins), and the heat is dissipated directly to the ambient air. Direct and indirect cooling systems operate without water loss (other than a small amount of water used to periodically clean the air-side surfaces of the air-cooled condenser or heat exchanger). The condensing temperature, in the case of direct dry cooling, or the cold water temperature, in the case of indirect dry cooling, is limited by the ambient air temperature, which is always higher than the ambient dry-bulb temperature. Although dry cooling achieves significant water savings, the capital and operating costs are much higher than they are for closed-cycle wet cooling, and the physical footprint is larger. Furthermore, plant performance is reduced in the hotter times of the year when the steam-condensing temperature (and hence the turbine exhaust pressure) is substantially higher (being limited by ambient air temperature) than it would be with wet cooling.
Another dry cooling system is the Heller System, which uses a direct contact condenser instead of a steam surface condenser. In this system the turbine exhaust steam is in direct contact with a cold water spray, and no condenser tubes are used. The resulting hot condensate and water mixture are pumped to an external air-cooled heat exchanger. The air-cooled heat exchanger may have a mechanical draft design, a natural draft design or a fan-assisted natural draft design. The direct contact condenser has the advantage of lower terminal temperature difference (TTS, which is the temperature difference between the saturation steam temperature and the cooling water outlet temperature), and thus lower turbine back-pressure.
Hybrid cooling systems have both dry and wet cooling elements that are used alternatively or together to achieve the best features of each system. In a hybrid cooling system a power plant can achieve the wet cooling performance on the hottest days of the year, and the water conservation capability of dry cooling at other times. The wet and dry cooling components can be arranged in series, or in parallel, and may be separate structures or integrated into a single tower. The dry cooling system elements can be either direct or indirect types. The most common configuration for hybrid cooling systems to date has been parallel, separate structures with direct dry cooling.
Like the wet cooling systems described hereinabove, the wet cooling elements of a hybrid system use significant amounts of water, particularly during the summer months. Therefore, it is most suitable for sites that have significant water availability but not enough for all-wet cooling at all times of the year. For sites where water use is highly limited or contentious, even the use of 20% of the all-wet amounts might be unacceptable, requiring all-dry cooling to allow the plant to be permitted. For sites with adequate water, the performance and economic advantages of all-wet cooling systems are significant. In some cases, plant siting might be eased by evidence of “responsible citizenship,” in which by means of a hybrid cooling system a plant developer offers some degree of reduced water use to the local community concerned about water for agriculture, recreation, or industry.
The disclosed technology overcomes the aforementioned problems associated with power plant condenser cooling. A broad object of the disclosed technology is to provide a novel method and apparatus for removal of waste heat from power plant condensers with high overall process thermal efficiency and without water waste.
Another object of the disclosed technology is to provide for power plant cooling in a relatively compact apparatus, by maximizing the thermal capacities of the apparatus. A further object of the disclosed technology is to provide a dry cooling system and method of dry cooling for effective heat removal or heat generation, operating at a high coefficient of performance.
General Description of the Disclosed Technology
In accordance with the above objects, the disclosed technology relates to cooling systems and methods which function to provide power plant condensers with return water at the necessary temperature levels to maintain power production at their optimum thermal efficiency levels. Optimum condenser temperature varies depending on the power plant's design and its geographic location. Condenser temperature design for combined cycle and steam power plants ranges between 35-52° C. As hereinabove discussed, the condenser's ability to lower supply water/condensate temperature determines the back pressure for the low-pressure steam turbine, wherein an increase in condenser temperature increases the back pressure on the turbine blades, leading to reduced power plant efficiency.
The disclosed technology may also replace the power plant condenser, or be used to improve other dry cooling systems. The disclosed technology further may be used in other applications, such as providing cooling and heating in air conditioning systems, and generally in the removal of heat from waste/stream heat sources.
The disclosed technology is specifically useful in a power plant's dry cooling system, using the depolymerization of a polymer over a catalyst in a closed system, including in liquid communication a plurality of heat exchangers configured to form depolymerization and polymerization assemblies. In some embodiments a cold energy storage assembly is also provided.
The depolymerization process and assembly of the disclosed technology depolymerizes a polymer over a catalyst, resulting in a monomer rich vapor. This depolymerization process is an endothermic reaction, drawing heat from the source water (e.g., condenser water or steam exiting the low pressure turbine, last stage) flowing through the heat exchanger in a depolymerization cooling unit (DCU).
The monomer rich vapor is then transferred to the polymerization assembly, reacting over an acid catalyst bed in a polymer heating unit (PHU) to convert the monomer back to the original polymer in liquid phase. The polymerization process is an exothermic reaction, and heat generated may be expelled from the heat exchanger vessel of the polymerization assembly by either air cooled or liquid cooled processes. In some embodiments, the polymerization assembly employs the dry cooling approach to expel heat from the PHU, using air cooled heat exchangers. To complete the cycle, the polymer stream is pumped by a liquid pump back to the DCU to provide below ambient wet bulb temperature cooling for a standalone cooling system.
To achieve continuous operation with high conversion efficiencies, the system may include one or more polymer separation units (PSU), whereby using heat from an independent stream of source liquid or ambient air, the monomer vapor rich mixture from the DCU and/or the polymer rich liquid mixture from the PHU are further separated into two streams: a vaporous light monomer rich stream and a liquid polymer rich stream. The PSU(s) thereby creates a buffer between the DCU and the PHU. In some embodiments a single PSU is placed downstream of the DCU and downstream from the PHU, enhancing polymer/monomer separation from each assembly. In another embodiment, a first PSU can be placed downstream of the DCU, enhancing polymer-monomer separation from the DCU product vapor stream, and a second PSU is placed downstream of the PHU, enhancing polymer-monomer separation from the PHU product liquid stream. In either of these configurations, the light monomer-rich stream from the PSU(s) is circulated into the PHU for further polymerization reaction, while the polymer-rich liquid stream from the PSU(s) is circulated directly to the DCU for depolymerization, or collected in a holding tank for later circulation through the DCU.
To provide cooling below ambient wet bulb temperatures during hot summer days with temperatures higher than the saturation temperature at steam turbine back pressure, the elevated temperature polymer produced in the PHU may be stored in a cold energy storage assembly, having a day storage tank (DST) which stores the elevated temperature polymer from the PHU (or the PSU). In the evening, the elevated temperature polymer cycles through a polymer cooling heat exchanger unit (PCU), dissipating its sensible heat into the cooler evening ambient air. The lower temperature polymer may then be stored in a cold energy storage tank (CST), where it waits for reuse the next day by pumping the liquid polymer to the DCU, and the cycle is repeated.
In some embodiments, water is incorporated into the depolymerization/polymerization cycle of the disclosed technology, partially vaporizing in the DCU with the depolymerization of the polymer, and condensing in the PHU with the polymerization of the monomer,
For optimal performance, the polymer should be selected based on the temperature range in which it depolymerizes and polymerizes, wherein in the power plant condenser cycle the temperature range of depolymerization is comparable with the power plant's cooling system operating temperatures, and the temperature range of polymerization exceeds the hottest ambient air summer temperatures at the site. Other temperature ranges may be suitable in other applications, and therefore other polymers may be more suitable.
In an exemplary embodiment of the disclosed technology, the liquid polymer is paraldehyde, which is depolymerized in the DCU into the light monomer acetaldehyde over an acid based catalyst. The acetaldehyde rich vapor, having a small amount of paraldehyde gas, is actively removed from the DCU as vapor using a blower, compressor or vacuum pump. This active removal of acetaldehyde rich vapor allows the paraldehyde to be depolymerized beyond its chemical equilibrium. The depolymerization and resulting vaporization process are endothermic, resulting in heat absorption from the source liquid flowing through the heat exchanger of the DCU. The maximum coolant specific energy, estimated based on 100% depolymerization conversion, is 1,434 kJ/kg. In practical operation, the depolymerization process can be controlled by varying operating parameters with high conversion up to 95%, providing a coolant specific energy up to 1,363 kJ/kg to meet cooling needs. This practical coolant specific energy is up to 4 times of the latent heat storage capacity of ice.
The monomer conversion of acetaldehyde in the polymerization process of the PHU is typically between 60-80%, depending on the process temperature (e.g., ambient air temperature for an air cooled heat exchanger). However, as hereinabove discussed remaining light and liquid monomer can be separated from the polymer rich liquid in the PSU, and excess light monomer can be recycled back to the polymerization assembly. With this recycling, the overall monomer conversion may reach 95%. Thereby, the exothermic process has polymerization conversions that match the depolymerization conversions for the endothermic process, allowing the cycle to be operated continuously and efficiently as a heat pump cycle by removing heat from the cooling process, and rejecting that heat from the heating process, with overall coolant energy density up to 1,363 kJ/kg.
The disclosed technology further provides a process for an efficient dry cooling system to dissipate low quality heat from chemical, mechanical, thermal, or power plant operations. It can work as a standalone system, or be synchronized with other dry cooling units. Further, it is contemplated that the exothermic polymerization process may be used as a heat source for other processes or purposes, such as for example a distillation unit.
The cycle of the disclosed technology operates based on chemical heat pump fundamentals and utilizes chemical thermal energy storage. Therefore, it is more tolerant to ambient temperature fluctuation than traditional dry cooling technology such as air cooled heat exchangers. For example, at an ambient temperature of 45° C., air cooling of a 45° C. water stream is impossible since there is no driving force for the heat transfer between water and air. With the cycle of the disclosed technology at the same ambient temperature condition, the endothermic process will lower the coolant/polymer temperature, allowing heat transfer between the water and the coolant. Using paraldehyde as the polymer, even under conditions when the coolant/polymer is fed at temperatures higher than the hottest ambient temperatures, the coolant performance will observe less than 1.4% performance penalty per 10° C. increase in polymer temperature. This behavior is caused by the small ratio between the paraldehyde sensible heat capacity and the overall reaction specific enthalpy change. Specifically, the sensible heat capacity for paraldehyde is 0.27 kJ/mol/C; therefore, the sensible heat storage for 10° C. temperature change is only 2.7 kJ/mol, which only accounts for 1.4% of total reaction heat (189.5 kJ/mol). For example the increase in the polymer temperature from 25-35° C. reduces the DCU cooling capacity by 1.4% (a polymer feed at 25° C. gives a DCU cooling capacity of 1 kW; when its temperature increases to 35° C., its cooling capacity is reduced to 0.986 kW). Similarly, the monomer will regenerate in the polymerization process with a process temperature higher than the ambient 45° C. temperature, allowing heat to be rejected to the environment using a traditional air cooled heat exchanger. Thus, the cycle of the disclosed technology allows the system to provide efficient cooling at high ambient temperatures, when traditional dry cooling methods fail.